Electrical capacitance or capacity is the ability of a body to hold an electrical charge. It is also a measure of the amount of electric charge stored by a capacitive device for a given electric potential. For example, a capacitive device may be any type of capacitor. However, any other electrical device may have a capacitance, at least a parasitic capacitance, since whenever an electric voltage exists between two separated conductors, an electric field is present between those conductors. Capacitive sensors utilize the fact that this electric field is sensitive for incidents, such as proximity change of an object, change of a distance between two conductors of the capacitive device (for example due to a pressure change) or change of the electric charge caused for example by a finger touch. For detection and measurement of such an incident, an accurate, reliable and fast measurement of the change in capacitance of the sensor may be required, even for very small capacitance values.
Small capacitances, for example capacitance changes due to a finger touch received at a touch sensor or any other capacitive touch solution, for example being part of a human touch interface, may for example be sensed using a common capacitance sensing circuit as illustrated in the schematic diagram of FIG. 1. The shown circuit comprises a pull-up resistor 12 having a high resistance, for example 1 MΩ, connected to a capacitive device 10 having a capacitance and an additional capacitance caused for example by a finger touch when using the circuit as part of a touch sensor application. Node 16 of the shown circuit is connected to a GPIO (general purpose input/output) pin 18 of a microcontroller (MCU) 20. The resistor 12 may be connected to the same voltage supply Vdd as the MCU 20.
Referring also to FIG. 2, a schematic diagram of a variation in time t of voltage U(V) at the capacitive device 10 is shown. The MCU applies a first voltage 68 that may for example correspond to a logical level LOG0 to node 16, followed by a second voltage 70 that may for example correspond to a logical level LOG1, senses a voltage U at the capacitive device 10 and measures a period of time 22, during which the voltage U increases to the second voltage level 70. With the measured time information 22 and resistance value of the resistor 12, the capacitance of the capacitive device 10 can be calculated. However, the measurement period of time 22 for a single sample may be long, for example 40 to 500 microseconds for a typical finger touch sensor, allowing for noise disturbance and instability and requiring considerable consumption of power.